1. Field of the Invention
The present invention relates to a positive electrode for a lithium-sulfur battery, and more particularly, to a positive electrode for a lithium-sulfur battery and which exhibits high energy density.
2. Description of the Related Art
Recently, the rapid development of smaller, lighter, and higher performance electronic and communication equipment has required the development of high performance and large capacity batteries to power such equipment. Lithium-sulfur batteries are of interest because they have the highest theoretical energy density, 2800 Wh/kg (1675 mAh/g), as compared to other batteries. In addition, sulfur is an abundant and inexpensive material, and is also environmentally-friendly.
Rauh (Rauh et al., J. Electrochemical Society, 126(4): 523 (April 1979)) discloses a carbon electrode and a sulfur-dissolved organic electrolyte. Peled (2) (Peled et al., J. Electrochemical Society, 136 (1989) 1621) discloses the use of various organic solvents. The cell consists of a lithium negative electrode, a porous catalytic current collector which is loaded with sulfur, and an organic solvent containing lithium polysulfide. However, a battery with high energy capacity cannot be fabricated with these methods due to the reaction between the organic solvent and the sulfur.
DeJonghe, Visco, and Oyama (DeJonghe, U.S. Pat. No. 4,833,048; Visco, U.S. Pat. No. 5,162,175; and Nature 373, 598–600, 1995 by Oyama) attempted to use an organo-sulfur positive electrode in batteries, but the organo-sulfur material has a poor theoretical capacity and a low reaction rate at room temperature.
U.S. Pat. Nos. 5,523,179 and 5,814,420 (Chu) teach an active sulfur (elemental sulfur, S8) having the highest theoretical capacity. The active sulfur can provide electrodes with a high active material density and capacity density, which results in a high-capacity positive electrode (1675 mAh/g of sulfur). Positive electrodes disclosed in these patents include an electrically conductive material that permits electrons to move between the active sulfur (the electrochemically insulator), and the electrically conductive material. These patents also disclose an ionically conductive material that permits ions to move between the ionically conductive material and the active sulfur.
One of the requirements for a binder used in lithium-sulfur batteries is a high adherence. Where the binder has a poor adherence, there needs to be an increase in the amount of the binder used. An increase in the binder results in a consequent reduction in the amount of positive active material in the positive electrode, thereby deteriorating the capacity of the resulting battery. Another requirement is good stability at temperatures at which the battery is used. That is, the binder preferably has a high boiling point, as a low boiling point limits applications of the battery. A final requirement is that the binder exhibits no reactivity with an electrolyte.